According to known practice a fuse is provided which is capable of interrupting all currents from the rated maximum interrupting rating down to the rated minimum interrupting rating and which is connected in series with a so-called weak link expulsion fuse which is specially designed to effect interruption of currents below the value of the miniumum interrupting current rating of the current limiting fuse. Obviously it is desirable to eliminate the practice of requiring the use of two fuses.
Another widely used system for maintaining low temperature operation of a fuse utilizing silver fusible elements utilizes the so-called Metcalf or M effect. In this type of fuse, a silver ribbon is modified by the placement of a small deposit of tin or tin alloy at one point on the silver ribbon to form an eutectic alloy with the silver to promote melting at that point on the ribbon when it reaches a temperature of approximately 230.degree. C. In the absence of the M effect, silver elements melt at a temperature of approximately 960.degree. C. Obviously melting temperatures of such a high order of magnitude without the eutectic effect are destructive to the fuse and are counter productive to desirable fuse operation. Where the M effect is utilized, the melting of the silver ribbon is localized at that point and the resulting arc and continued current flow must increase the ribbon temperature by an additional 700.degree. C. approximately. In addition non-melting current flows can cause the alloy formation at the M spot to produce a permanent change in the fuse melting characteristic.
In one modification of the eutectic design, a parallel slave element is provided for the purpose of initiating two further breaks in the fusible element following the initial establishment of melting at the M spot. Such structure limits the points of melting to three and obviously is not altogether desirable and also introduces a degree of complication.
In accordance with another practice, a core is provided on which the fusible elements are wound and is constructed of gas evolving material. Where this type of structure is used venting of the housing is required. If the housing is vented of course the interrupting operation is not isolated and can result in failure of the fuse or damage to other apparatus.
Still another type of fuse utilizes a silver element connected in series with a tin element. The tin element is enclosed in an insulating tube and is expelled from the tube into the filler element to achieve low current interruption. Obviously this structure involves a measure of complication, and in addition is only suited for lower current ratings.
Still another practice has involved thermally insulating a silver wire section arranged in series with a silver ribbon. The heat concentration promotes earlier melting of the silver wire. It adds substantially to the cost of the fuse.
Still another practice has involved the use of a gold alloy in an arc quenching tube connected in series with a silver element so as to aid in the interruption of low currents.
From the above discussion of prior practices, it is evident that there are difficulties involved in interrupting low values of current. Furthermore the requirement for interrupting low currents has added substantially to the complexity of fuse designs, to their size and cost. It also limits their maximum current ratings and their application.
Cores on which fusible elements have been wound are known but are objectionable because contact with the fusible element reduces the area over which energy exchange between the arcs and the filler material can take place. Since the interrupting process requires that most of the arc energy be transferred to latent heat of fusion of the filler material, any reduction of the area of contact with the filler material is undesirable.
Areas of contact between the elements and core can produce high temperatures in the core. Ceramic materials exhibit marked reduction in their insulating properties at such elevated temperatures. This reduction in insulating property of the core results in a non-uniform voltage distribution across the fuse in the period following arcing.
Under certain transient current conditions, an appreciable temperature rise in the fusible elements may occur and may effect a deformation of the fusible elements. Repeated heating and cooling cycles may impose increasing tensile load on the fusible elements since they may not straighten out due to constriction of the sand. If movement of the elements is possible, tension may be relieved. In elements wound on a core, opportunity for relieving tension is severly restricted and mechanical failure due to tension may occur since increases in tension may break the fusible element particularly at the points of reduced cross section.